Directions: In this question, you need to replace the bold part of the sentence by the most suitable idiom/expression given as option.
Maths is the only subject that seems extremely difficult for me to understand.
» Explain it
A piece of cake (idiom): Something very easy.

All greek to me (idiom): Something difficult to understand.

Ducks and Drakes (Idiom): Behave recklessly

Ace in the hole (Idiom): A hidden strength.

Clearly option B is the most suitable idiom.

Hence option B is correct.
» Explain it
As busy as bee (idiom): Occupied with lot of work.

At the drop of a hat (idiom): without any hesitation.

Barking up the wrong tree (idiom): looking at the wrong place.

Clearly option A is the most suitable idiom.

Hence option A is correct.
Harish Salve is an important and powerful lawyer as he wins every court case he gets.
» Explain it
A jack of all trades (Idiom): A person who has dabbled in many skills.
A bit shot (Idiom): An important or influential person.
A big gun (Idiom): An important or influential person.
Clearly, both option B and C are replacing the bold part appropriately. 
Option D is hence the correct answer.
I have to work late night only then this task will be finished on time.
» Explain it
Cut Corners (Idiom): Do something to save money.

Cut the mustard (Idiom): To come upto expectations.

Burn Midnight oil (Idiom): To work late night.

Hence option C is correct.
» Explain it
Hitting the sack (Idiom): going to bed.

feeling a bit under the weather (idiom): Feeling sick

making a short story short (idiom): to come to the point.

The last straw (idiom): The final problem in a series of problems.

Hence option B is correct.